Originally named after one of the most disgusting people alive, the reagen is the very end of the beer left in the bottle or can, that is warm and contains backwash. The name for this disgusting bit of liquid was conceived one night in which Reagan Gilroy actually went around and drank all of the reagens in the house.

George: "Hey Bill, I dare you to chug an entire funnel of all the reagens left over..."
Bill: "Hell no, I wouldn't do that for $100 dollars. You gotta be some sick bitch to drink that shit."